1. Field of the Invention
The present technique relates generally to production of polyphenylene sulfide (PPS). In particular, the present technique relates to determining water content in a PPS reactor based on reactor variables.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Polyphenylene sulfide (PPS), also known as poly(arylene) sulfide, is a high-performance engineering thermoplastic that may be heated and molded into desired shapes in a variety of manufacturing, commercial, and consumer applications. PPS may be used in the preparation of fibers, films, coatings, injection molding compounds, and fiber-reinforced composites, and is well-suited for demanding applications in appliance, automotive, and electrical/electronic industries. PPS may be incorporated as a manufacturing component either alone or in a blend with other materials, such as elastomeric materials, copolymers, resins, reinforcing agents, additives, other thermoplastics, and the like. Initially, PPS was promoted as a replacement for thermosetting materials, but has become a very suitable molding material, especially with the addition of glass and carbon fibers, minerals, fillers, and so forth. In fact, PPS is one of the oldest high-performance injection-molding plastics in the polymer industry, with non-filled grades commonly extruded as coatings.
PPS polymer, including semi-crystalline PPS, is an attractive engineering plastic because, in part, it provides an excellent combination of properties. For example, PPS provides for resistance to aggressive chemical environments while also providing for precision molding to tight tolerances. Further, PPS is thermally stable, inherently non-flammable without flame retardant additives, and possesses excellent dielectric/insulating properties. Other properties include dimensional stability, high modulus, and creep resistance. The beneficial properties of PPS are due, in part, to the stable chemical bonds of its molecular structure, which impart a relatively high degree of molecular stability, for example, toward both thermal degradation and chemical reactivity.
The general molecular structure of PPS is a polymer composed of alternating aromatic (phenylene) rings and sulfur atoms (in a para substitution pattern), as shown below.
The molecular structure may readily pack into a thermally stable crystalline lattice, giving PPS that is a semi-crystalline polymer with a high crystalline melting point of up to about 285° C. and higher. Because of its molecular structure, PPS also tends to char during combustion, making the material inherently flame retardant, as mentioned. Further, the material will typically not dissolve in solvents at temperatures below about 200° C.
Though PPS was first discovered in the late 19th century, many engineers failed in their attempts over the years to produce PPS for industrial use, and thus the history of PPS as an industrial material is relatively short. In 1967, however, Phillips Petroleum Company of Bartlesville, Okla. devised a method for producing PPS through the synthesis of para-dichlorobenzene and sodium sulfide, as described below.

This condensation polymerization (or step polymerization) marked the beginning of industrial-scale commercialization of PPS. In 1972, Phillips Petroleum Company began commercial-scale production of PPS, and this PPS was soon noted for having an effective balance of thermal and chemical resistances, nonflammability, and electrical properties. Today, PPS is manufactured and sold under the trade name Ryton® by Chevron Phillips Chemical Company LP of The Woodlands, Tex.
In general, PPS may be prepared by reacting a dihalogenated aromatic compound with a sulfur source under polymerization conditions in the presence of a polar organic compound. The polar organic compound, such as N-methyl pyrrolidone (NMP), is generally an organic solvent that maintains the reactants and PPS polymer in solution during the polymerization. A molecular weight modifying agent, such as an alkali metal salt, may be optionally added to the polymerization mixture. Typically, the polymerization reaction mixture comprises aqueous and organic phases, with the PPS polymer dissolved primarily in the organic phase. Generally, after the majority of reactants have polymerized, the reaction mixture may be cooled to terminate the polymerization and to drop the PPS polymer solid from solution. Such cooling of the polymerization may be accomplished, for example, by reducing the pressure of the reaction mixture to flash the polar organic compound (e.g., NMP), or by adding more NMP to the mixture to cool (quench) the mixture. The choice of flashing the existing NMP or quenching with more NMP may depend upon the design of the particular manufacturing plant, as well as the particular grade of PPS. Moreover, the choice may affect the process economics, as well as the polymer bulk properties, morphology, particle size, and the like.
Another process alternative in the termination step is to cool (or quench) the polymerization by adding water to the reaction mixture. A water quench, relative to an NMP quench, typically results in a larger particle size of the PPS, which may facilitate separation of the PPS product from undesirable solid components formed in the polymerization since the undesirable components, e.g., residual salt and slime, typically have a relatively small particle size. A problem with water quench, however, is that if too much water is added, the PPS particle size (average diameter) may become too large for downstream separation/handling equipment, resulting in damage or shutdown of the equipment, off-spec production of PPS, contamination of the PPS, and so forth. Conversely, if too little water is added, the PPS particle size may be too small, resulting in losses of PPS escaping with the separated stream of undesirable components.
To complicate matters, the amount of water existing in the reactor immediately prior to quench varies and is typically unknown. Furthermore, it is the total amount of water in the reactor, and not just the amount of quench water added, that affects the PPS properties. Water may exist in the reactor prior to quench because of inefficiencies in the upstream dehydration of the feedstock entering the reactor and because water may be a product of the PPS (condensation) polymerization in the reactor.
General correlations are known between the total amount water in the reactor during quench versus the generated PPS particle size, but again, the determination of how much quench water to add is problematic because the amount (and concentration) of existing water is typically unknown. In the PPS manufacturing process, the human operator typically guesses, based on experience, trial-and-error, “feel” of the operating conditions, and so forth, as to how much water exists in the reactor and as to how much quench water to add.
It should be noted that laboratory or on-line sampling of the reactor mixture to test for the water content may be problematic due to the harsh reactor conditions. Further, it may be difficult to obtain a representative sample of the reaction mixture which may comprise partially-dispersed aqueous and organic phases. Also, testing may be expensive and time-consuming. Moreover, during sampling and analysis, the polymerization may proceed and conditions may change, sometimes undesirably.
Lastly, it should be explained that the PPS polymer may remain substantially dissolved in the reactor solution even after the quench liquid is added. In this case, after the quench liquid is added, the reactor contents may be cooled with a reactor coolant system to precipitate the PPS. If the right amount or type of quench liquid is not added initially, the PPS particles that drop from solution during the controlled cooling may not be the desired size. Generally, there is not a second chance to adjust the amount of quench water or the particle size of the PPS polymer.
In conclusion, the determination of the amount of water existing in the reactor prior to quench is problematic because, in part, other liquid components, such as NMP, are present. Thus, a conventional volumetric measurement, for example, such as through the use of reactor level indication, gives the volume of the mixture and not just the volume of the water. There is a need, therefore, for a technique to determine the amount of water existing in the reactor prior to quench (or cool down) of the reaction. The technique should further determine how much quench water to add to the reaction mixture to control the total amount of water in the reactor during quench to give the desired particle size and other properties of the PPS.